Conventionally, raw materials for the main soft segment of a polyurethane that is produced on an industrial scale are classified into an ether type typified by polytetramethylene glycol, a polyester polyol type typified by adipate-based ester, a polylactone type typified by polycaprolactone, and a polycarbonate type typified by polycarbonate diol (Non-Patent Document 1).
Of these, a polyurethane using an ether type is excellent in the hydrolysis resistance, flexibility and stretchability but is inferior in the heat resistance and weather resistance. A polyurethane using a polyester polyol type suffers from low hydrolysis resistance of the ester group and cannot be used depending on the usage, though the heat resistance and weather resistance are improved.
A polyurethane using a polylactone type is considered to be of the grade excellent in hydrolysis resistance, compared with a polyurethane using a polyester polyol type, but due to likewise having an ester group, the hydrolysis cannot be completely suppressed. In addition, it has been proposed to mix these polyester polyol type, ether type and polylactone type and use the mixture as a raw material of a polyurethane, but complete compensation for defects in respective types is not achieved.
On the other hand, a polyurethane using a polycarbonate type typified by polycarbonate diol is considered to be of the best durability grade in terms of heat resistance and hydrolysis resistance and is widely used as a durable film, an artificial leather for cars, an (aqueous) paint, and an adhesive.
The polycarbonate diol widely available on the market at present is mainly a polycarbonate diol synthesized from 1,6-hexanediol, but this material has high crystallinity, imposing a problem that when processed into a polyurethane, the cohesion of soft segment is high and among others, low-temperature characteristics such as flexibility, elongation, bending or elastic recovery performance at low temperatures are poor, and for this reason, the application is limited. Furthermore, it is also pointed out that an artificial leather produced using this polyurethane as a raw material has low chemical resistance, hard touch and, compared with natural leather, poor “texture”.
In order to solve these problems, polycarbonate diols having various structures have been proposed.
For example, a method of making a copolymerized polycarbonate diol by using 1,6-hexane diol and another dihydroxy compound as raw materials is known, and specifically, there have been proposed a polycarbonate diol in which 1,6-hexanediol and 1,4-butanediol are used as raw materials and copolymerized (Patent Document 1), a polycarbonate diol in which 1,6-hexanediol and 1,5-pentanediol are used as raw materials and copolymerized (Patent Document 2), a polycarbonate diol in which 1,4-butanediol and 1,5-pentanediol are used as raw materials and copolymerized (Patent Document 3), and a polycarbonate diol in which 1,3-propanediol and another dihydroxy compound are used as raw materials and copolymerized (Patent Document 4).
A method using a dihydroxy compound having a substituent on the main chain has also been proposed as a powerful method to inhibit crystallinity of a dihydroxy compound-derived moiety, and, for example, there are a polycarbonate diol in which 2-methyl-1,3-propanediol and another alkylene glycol are used as raw materials and copolymerized (Patent Document 5), and a polycarbonate diol in which 3-methyl-1,5-pentanediol and another alkylene diol are used as raw materials and copolymerized (Patent Document 6).
In addition, a polyurethane using a polycarbonate diol in which a long-chain diol, such as 2-methyl-1,8-octanediol or 1,9-nonanediol, is used as a raw material, has been proposed (Patent Document 7).
Furthermore, a polycarbonate diol in which the average carbon number of a diol exceeds 6, has been proposed as a polycarbonate diol excellent in the chemical resistance, low-temperature characteristics and heat resistance (Patent Document 8).
A polyurethane using a polycarbonate diol, of which flexibility, chemical resistance, etc. are improved by the mixing and use of a polycarbonate diol using a diol having a carbon number of 3 to 6 as a raw material and a polycarbonate diol using a diol having a carbon number of 7 to 12 as a raw material, has also been proposed (Patent Documents 9 to 11).
A polycarbonate diol using only 1,4-butanediol as a raw material of a dihydroxy compound has also been proposed (Patent Documents 12 to 14).
On the other hand, as for the production method of a polycarbonate diol, a proposal has been made about a method for synthesizing a polycarbonate diol having a high molecular weight, and there are described a method of reacting a dihydroxy compound with an alkyl carbonate to make a polycarbonate diol having a molecular weight of 500 to 2,000 and further adding an aryl carbonate to produce a polycarbonate diol having a molecular weight of more than 2,000 (Patent Document 15), and a method of reacting a dimethyl carbonate and an aliphatic dihydroxy compound to produce a less colored and high-quality polycarbonate diol in which the molecular terminal is a hydroxyl group (Patent Document 16).